SMPS use switching converter ICs. Some of these ICs do not comprise a built-in power switching device at the primary side. Such ICs usually have the following five function pins or terminals:
(a) Vdd—positive power supply terminal;
(b) Vss—negative power supply, or return terminal;
(c) FE—feedback information terminal, which receives error information from the secondary side. The duty cycle of the main switching device is adjusted according to information received at this terminal;
(d) OC—over-current limit terminal for controlling the maximum current through the power switching device for conversion of energy to the secondary side; and
(e) DR—driving terminal as the driving source for controlling the switching duty cycle of the power switching device.
A typical block diagram for such an IC 10 is shown at FIG. 1. A typical application circuit diagram using a bipolar power switching device is shown at FIG. 2 and a typical application circuit diagram using a MOSFET power switching device is shown at FIG. 3.
In FIG. 1, the power-up and reference generator block 11 controls the power-up of the IC, provides operating voltage (typically regulated) to other functional blocks and generates various reference levels (voltage or current) for other circuit blocks. The output duty cycle control circuit block 12 adjusts the duty cycle at the DR terminal according to the signal received from the FE terminal and hence maintains the output of the switching converter application circuit (see FIGS. 2 and 3) at the target value.
The driver block 13 provides sufficient driving for the main switching device 14. The OC terminal samples the current through the main switching device 14, and shuts down the DR terminal in case this exceeds the target over-current limit.
Because the typical switching converter ICs as described above have five terminals this restricts the packaging options and low-cost standard package types with fewer than five terminals cannot be exploited.